In the manufacture of certain electronic components such as integrated circuits and transistors, extraordinarily pure specimens of silicon of germanium are required. The electronic industry uses the technique known as zone refining to separate these pure specimens. In 1952 William G. Pfann of the Metallurgical Research Department of Bell Telephone Laboratories developed this process for the purification of materials used in the semiconductor industry. Pfann found that by passing a molten zone along a rod of silicon or germanium, that is a varying segment of the rod that is heated to a liquid state, he could cause the impurities to move in the direction of the molten zone. The impurities were reduced in the solid parts of the rod to a few parts per billion, by repeating this process many times. Pfann found that the impurities were concentrated at the end of the rod. This process is now widely used in the semiconductor industry as a purification technique.
After the initial development by Pfann, chemical researchers used the principles of zone refining for the purification of organic chemicals, with some success. When an organic liquid is cooled crystals are formed. The researchers found that when a homogeneous organic liquid mixture is cooled, the composition of the solid that crystallizes in the mixture has a lower concentration of impurities than that of the surrounding liquid. The purification of a substance by zone refining depends on this concentration differential between phases, since the liquid retains much of the impurities.
Early colonial Americans made use of a primitive form of zone refining when they left jugs of fermented apple juice or hard cider outside on cold nights. The water in the jugs would freeze out of the mixture, leaving only the alcohol and other impurities in liquid phase. These impurities could then be poured off the ice the next morning. Repeating this process for several nights increased the concentration of alcohol to high levels.
The current chemical laboratory method for zone refining organic materials is to pass a molten zone, that is a zone of heat sufficient to melt organic crystals into liquid phase, across a chemical sample contained in a vertically oriented glass tube. In this fashion a varying portion of the glass tube is heated throughout a radial segment. The diameter of the glass tubes used in this process is usually between 5 mm and 20 mm and the length is typically between 15 cm and 50 cm. The most common means of heating the tube is by passing an electric current through a resistance wire loosely wrapped about the tube. The wire is attached to a mechanism which moves it along the length of the tube at a rate of between 0.1 cm and 20 cm per hour. The passage of the resistance wire along the length of the tube creates a moving heated zone relative to the tube.
Experimental problems have been associated with purifying organic chemicals by the vertical glass tube method however. These problems are caused by the relatively low thermal conductivity of organic compounds and by the coefficient of thermal expansion of organic materials being many times higher than that of glass. If a careless operator applies too much heat to a sample, the sample expands and bursts the glass tube. The glass tubes also limit the rate that the heated zone can be moved, since the heat transfer rate into the middle of the tube is slow. Another limitation of this method is in the quantity of sample that can be refined at any given time. If the tube diameter or length is lengthened to increase capacity, the time required to pass a heated zone from one end to the other is increased proportionately.
The zone refining methods used in current chemical laboratories therefore require a great deal of attention to experimental technique. The purification obtained often requires days to achieve. These limitations have prevented an otherwise promising experimental technique from becoming a widely practiced laboratory procedure.
Zone refining resembles distillation and gas and liquid chromatography in that it depends upon the distribution of material between two phases. The theory developed for fractional distillation and gas and liquid chromatography can be applied directly to zone refining with some modifications. Both theories predict that the highest purity and best separation is achieved when a large number of separation stages i.e., plates in distillation, and theoretical plates in chromatography, are involved. These theories also predict that the speed of the separation can be significantly increased if the dimensions of the separating apparatus are made sufficiently small. When objects are small, the rates of heat and mass transfer in and out of the object is much faster than with large objects. It would therefore be expected that the highest efficiency in zone refining would be obtained if the tubing were made very small and the number of zones increased so that the sample is subjected to several zones at one time. Obviously however, extremely small tubes can contain only extremely small amounts of sample i.e a few milligrams. These small tubes would not be very useful to the chemist who requires a relatively large chemical sample for testing at a later date.
Given the practical limitations on conventional zone refining laboratory procedures, rapid high volume zone refiners have not been readily available. Accordingly, a need remains for a high speed zone refiner which allows for the purification of a relatively large sample of organic substance over a relatively short time period. Furthermore, a need remains for a commercially viable zone refiner which is not susceptible to the problems of traditional vertical glass tube refiners. Accordingly, it is to the provision of such that the present invention is primarily directed.